On the diversity of reversible products of phototransformations in ethanol solutions of rhodamines at stepwise quantum excitation

The formation of various reversible products of phototransformations (RPP) in rhodamine 6G (R6G), rhodamine C (RC), and unsubstituted rhodamine (UR) solutions at pulsed stepwise quantum excitation is investigated. The dependence of the type of RPP on the specific system of molecular orbitals (MO), within the bounds of which light quanta are absorbed, is revealed. The excitation of molecules in the MO system responsible for absorbing the light with the wavelength λ = 353 nm leads to formation of RPP, mainly as semioxidized forms of R6G, RC, an UR with a lifetime τ = 10-10-10-9 s. The excitation of molecules in another system of orbitals, namely, in the system responsible for absorbing the light with λ = 532 nm, is accompanied by formation of RPP with τ = 10-7-10-6 s in the form of lactone modifications of RC and UR. Under similar conditions of exciting R6G molecules, the dye is converted not to the lactone form, but to another type of RPP with the same lifetime and with a much larger activation energy for its formation. The prospects of applying RPP in rhodamine solutions for the real-time optical control of spectral properties of these solutions are evaluated. 14 refs